A straightforward approach for temperature measurement is to place a thermometer in thermal contact with an object to be measured and allow the combination of thermometer and object to come to a thermal equilibrium characterized by uniform temperature. In this simple case, the object temperature can be read from the thermometer. Temperature measurement is more challenging when one or more of the preceding steps are impossible or impractical. For example, it is often desirable to measure temperatures at locations where it is undesirable (or even impossible) to place thermal sensors.
For example, in gun tubes, the drilling of a hole in which one inserts an electrical or optical thermal probe can damage the gun. The probe has to survive an extremely hostile environment and its re-use may be in question. For measurements of temperature in liquids, the probe has to be inserted into the liquid. This can contaminate the liquid and for very high resolution, produce errors in the measurement. The errors come from thermal paths that inevitably accompany the wires and sensor. Furthermore, some mediums are highly damaging to sensors, such as high temperature shocks, acids and other reactants. A sensitive gauge can be quickly destroyed in such an environment unless it is protected or coated. Such protection tends to introduce errors in the measurement. A thermal sensor can also undesirably alter normal medium dynamics. For example, in aerodynamic flows the presence of a thermal sensor on a surface can perturb air flow past the surface, thereby bringing the temperature measurements into question. Various remote temperature sensing methods have been developed in order to address such applications.
One class of remote temperature sensing methods is based on the use of acoustic radiation as a temperature probe. The basic physical effect exploited in such methods is the temperature dependence of an acoustic wave propagation velocity (either phase velocity or group velocity). For example, U.S. Pat. No. 6,834,992 considers an acoustic pyrometer, where an average temperature along a path (e.g., a path passing through a flame) is measured by propagating an acoustic signal having a short rise time along the path. A transit time of the acoustic signal is determined by comparing the transmission time of the rising edge with the reception time of the rising edge. An average temperature is determined from the transit time. Another example is U.S. Pat. No. 5,214,955, which considers a phase lock loop system for measuring temperature induced changes in acoustic phase velocity. Further examples of acoustic remote temperature sensing are considered in U.S. Pat. Nos. 4,353,256, 5,469,742, 6,378,372, and 6,481,287.
A further refinement of acoustic remote temperature sensing is considered in U.S. Pat. No. 4,513,749, where localized acoustic remote temperature sensing is provided by the use of two overlapping acoustic beams, one focused and the other unfocused. The relative phase between the two beams depends on the acoustic properties at the focal region of the focused beam, and is insensitive to acoustic properties elsewhere. Since the relative acoustic phase is only affected by temperature changes in the focal region, localized remote temperature sensing is provided.
Most conventional acoustic remote temperature sensing methods rely on having a constant temperature along the acoustic propagation path. In such cases, the relation between an acoustic path delay and the path temperature is straightforward. For example, reflections of acoustic pulses from marks on a rod have been employed. The rod is the thermal sensor (due to its thermal expansion) and must be placed into the medium to be measured. Other ultrasonic thermometers are based on the resonance frequency of a thin disk. Again, the thin disk must be placed into the medium of interest. Neither of these devices can be used accurately unless they are at constant temperature. However, in some cases of interest (e.g., when measuring a dynamic change in temperature), the temperature will not be uniform along the acoustic propagation path.
Some remote thermal sensors measure a rate of temperature change, which when combined with knowledge of the time constant of the sensor can determine the end-point temperature at a remote location from the slope of the temperature rise. Such devices tend to have a time lag until the thermal sensor first sees a change in temperature, due to (relatively slow) thermal transport from the remote location to the sensor. Mounted to a thick gun tube, such a sensor cannot see the initial super-hot temperature created by an explosion. Instead, it can only see the influence of that heat pulse after it diffuses through the thickness of the tube.
Accordingly, it would be an advance in the art to provide acoustic remote temperature sensing that does not require a constant path temperature, that is suitable for dynamic temperature measurements, and that is not limited in response time by slow thermal transport.